The invention relates to a process for autofrettage of thick-walled workpieces according to the features in the preamble of claim 1 and a device for carrying out the process.
Autofrettage refers to a process for increasing the static and dynamic load of thick-walled components that are subjected to an interior pressure. With this process, internal compressive stresses are intentionally built up by applying a stretching pressure that far exceeds the operating pressure, and wherein the yield strength of the material is partially exceeded. Partial regions of the material are permanently plastically deformed under the high pressure, whereas the exterior regions of the workpiece are only elastically deformed. This produces a pretension inside the workpiece and not only improves the operating strength, but preferably produces high-yield-strength components with a long service life, as required in particular for diesel injection. The so-called common rails, i.e., recently developed fuel distribution rails, are exposed to pressures in the order of 200 MPa and above during normal operation. The same applies to lines carrying fuel in the vicinity of the common rail and to connected injectors.
Typically, during autofrettage, a high-pressure fluid is introduced into the workpiece to be treated and exposed to high-pressure, wherein a pressure generator arranged outside the workpiece compresses the high-pressure fluid which is in fluid-connection with the interior of the workpiece via a connecting line. Typically, pressure boosters with a high transmission ratio are employed. For example, the transmission ratio, which can be set equal to the ratio of the diameters of the pistons of the pressure booster, can be greater than 1:30. Because pressure of several hundred MPa is applied on the high-pressure side, particular measures must be taken in order to ensure fluid-tightness of all components of the assembly, in particular the high-pressure piston. This is accomplished by guiding the high-pressure piston in a high-pressure cylinder which is made, for example, of a hard metal. Hard metals are brittle due to their high hardness and are sensitive to impact and shock loads. The high pressure cylinders made of a hard metal are therefore surrounded by additional thick-walled steel cylinders, which also pretensioned so as to be able to absorb the high forces in the high-pressure regime. This design increases the cost of the high-pressure cylinder, in particular because a hard metal cylinder representing a hard metal component is difficult to machine and also has a relatively large volume. It is also difficult to couple the hard metal cylinder to the component for the autofrettage process, because the lines from the high-pressure cylinder into the interior space of the workpiece must also be able to withstand the high pressures. Corresponding seals need to be provided, so that the entire structure becomes very complex. All components exposed to high-pressure, including the high-pressure cylinder, are basically subjected to wear and must therefore be exchanged sooner or later.
DE 41 15 284 A1 describes an autofrettage apparatus for pipes having two holders adapted to receive a pipe. In this apparatus, an arbor is provided that can be inserted into the pipe and has sealing elements for sealing an annular space located between the arbor and the interior wall of the pipe that is exposed to the pressurizing means. The pressurizing means are supplied through an arbor holder receiving the arbor. The arbor is provided to reduce the quantity of pressurizing means required for autofrettage by the constant volume corresponding to the arbor volume.